It is an offence to open someone elses mail, (this includes ex-wives). If a person receives mail which is not addressed to them, then the item must be returned. Either to the Post Office, or to the sender if there is an address on the back, marked 'Not at This Address' in both instances. Your partner should report this to the bank concerned, with a suitably worded complaint in view of the fact that the bank does have his new address. A letter from your partner's solicitor to the ex-wife regarding any mail received at her address, which she is not entitled to open, might also be of help.